"Gas-liquid-solid" (GLS) reactors of this type are widely used in industry, inter alia for the hydrogenation and (partial) oxidation of liquid phases, as well as for Fischer Trops reactions. In processes of this type in general the gas phase and liquid phase fed to the reactor contain the reactants and the products formed by the chemical reaction, whilst the solid phase is a catalyst material.
The GLS reactors currently used in industry are subdivided into fixed bed reactors and suspended solid phase reactors.
In the fixed bed reactors, the catalyst particles are large in size and are present in a fixed bed, through which the gas phase and the liquid phase are fed (simultaneously). The fixed bed reactors can be further subdivided into so-called "trickle bed" reactors, "co-current-upflow" reactors and "segmented bed" reactors, depending on the respective direction(s) of the liquid stream and the gas stream.
In the suspended solid reactors, small solid particles are used which are held in suspension in the liquid phase, for instance by stirring or mixing. Examples of reactors of this type are suspension reactors or fluidized bed reactors or stirred suspension reactors.
Examples of said known types of GLS reactors and specific applications thereof are given in Shah, Y. T., Gas-liquid-solid reactor design, McGraw-Hill, New York 1979.
General problems experienced when carrying out GLS reactions in said known reactors are, inter alia:
realization of good contact or good transfer between the various phases, by means of which a rapid and/or high degree of conversion, as well as a short contact time and/or residence time in the reactor, can be obtained; PA1 holding the solid phase in the reactor so that said phase is not entrained by the (liquid or gaseous) product streams removed from the reactor. If the solid phase is removed from the reactor with the product stream, as in the case of suspension reactors, downstream separation steps for separating off the solid phase and means for recirculating the solid phase to the reactor are required; PA1 maintaining a good flow through the reactor, that is to say preventing the solid phase present in the reactor from blocking the feed or the discharge for the liquid phase and/or the gas phase to/from the reactor; PA1 providing an adequate contact surface on the solid phase; PA1 achieving optimum control of the reaction conditions (including homogeneous temperature distribution) and the degree of conversion; PA1 the existing GLS reactors as a rule require a downstream separation step, in which the reaction products are separated off from the product stream removed from the reactor--which product stream contains the other constituents of the reaction mixture; PA1 it is not possible selectively to remove reaction products from the reaction mixture and/or the reaction zone in such a way that in the case of equilibrium reactions the equilibrium is shifted to the desired product side and a higher yield and/or a higher efficiency is obtained. PA1 a housing (1) which encloses an essentially closed reaction chamber (2) for holding a first liquid phase, the solid phase and a gas phase (fed through said chamber); PA1 at least one feed (3), connected to the reaction chamber (2), for feeding a first liquid phase to the reaction chamber (2); PA1 at least one hollow fibre (4), the wall of which defines an internal flow channel (5) for at least removing a second liquid phase, such that exchange of matter can take place between the reaction chamber (2) and the flow channel (5) through the wall of hollow fibre (4); PA1 at least one discharge (6), connected to the flow channel (5), for removing the second liquid phase; PA1 at least one gas inlet (7) and at least one gas outlet (8), connected to reaction chamber (2) such that, in the use position, the at least one gas inlet (7) opens essentially into the bottom of the reaction chamber (2) and the at least one gas outlet (8) is located essentially at the top of the reaction chamber (2), for feeding the gas phase essentially in the upward direction through the reaction chamber (2). PA1 holding the solid phase in the reaction chamber (2); PA1 feeding at least the first liquid phase through the at least one feed (3) to the reaction chamber (2); PA1 feeding the gas phase to the reaction chamber (2) through the at least one gas inlet (7) and removing the gas phase through the at least one gas outlet (8); PA1 removing the second liquid phase through the flow channel (5) of the at least one hollow fibre (4) to the discharge (6); PA1 under conditions such that at least one chemical reaction takes place in the reaction chamber (2) and that at least part or at least one constituent of the first liquid phase passes through the wall of the hollow fibre (4) and is removed with/as the second liquid phase through flow channel (5) to discharge (6).
A first aim of the present invention is, therefore, to provide an improved reactor for carrying out gas/liquid/solid reactions, in particular a GLS reactor of broad applicability, which is easier to operate and which makes it simpler to maintain the solid phase than, for example, (is the case with) suspension reactors.
A further aim of the present invention is to provide an improved GLS reactor which has fewer or none of the above-mentioned disadvantages.
It is known in the prior art to use hollow fibres for separation or absorption techniques, such as microfiltration, ultrafiltration and reverse osmosis, as well as dialysis, gas separation, pervaporation/vapour permeation, and installations for this purpose are described in the prior art. These processes generally relate to separation processes in which only gas phases and/or liquid phases are involved.
For instance, German Offenlegungsschrift 43 08 697 and Chemical Abstracts, Vol. 92, No. 14 (1980) describe systems for bringing liquid streams into contact, for example for mass transfer. Those systems comprise membrane modules, the first liquid stream being fed through the fibres and the second liquid stream being fed over the fibres. However, this system is not suitable for use as a GLS reactor; for instance, there is no specific reaction chamber, there is no system for holding the solid phase in suspension and there are no separate feeds for feeding the liquid phase and the gas phase to the reaction chamber.
Membrane reactors are also used for gas phase reactions, in particular gas phase reactions which are catalyzed by a catalytic solid phase which forms the walls of the fibres or is immobilized on the hollow fibre, the latter acting as a support. A system of this type is described, for example, by G. Saracco et al., Journal of Membrane Science, Volume 95, No. 2, October 1994, p. 105-123.
However, a liquid phase is not involved in reactions of this type, nor is it possible to carry out phase transfer reactions. Furthermore the catalytic solid phase is present as or on the membrane, which appreciably restricts the number of catalysts which can be used--and thus the number of reactions which can be carried out.
A process recently developed by the Applicants is membrane gas absorption. With this process constituents are absorbed from a gas phase into a liquid phase, the gas phase and the liquid phase being held separate by a (semi-permeable and/or porous) membrane. Although chemical reactions, by means of which the substances to be absorbed are bound, can take place during absorption of the desired constituents in the liquid phase used as absorbent, this technique cannot be used for GLS reactions, inter alia because with this process the gas phase and the liquid phase must be kept strictly separate. Furthermore, in membrane gas absorption the chemical reaction takes place only after the substance(s) to be absorbed has/have passed through the membrane, i.e. into the lumen of the hollow fibres.
R. Brambach and N. Rabiger, Chemie-Ingenieur-Technik, Volume 66, No. 3, March 1994, pages 362-365 describe a biological reactor for converting substances which are difficult to biodegrade. This reactor comprises a suspension membrane reactor consisting of a multiplicity of membrane modules, wherein each module "aus einer der Reaktionsraum umschliessenden Tubularmembran aus Polyacrylnitril d.sub.1=14.2 mm mit A=0.05 m.sup.2 besteht" ("consists of a tubular membrane of polyacrylonitrile, d.sub.1=14.2 mm where A=0.05 m.sup.2, which encloses the reaction chamber"). This reactor is supplied with compressed air via a distributor unit at the bottom.
However, the reactor in this article is a biological reactor and not a three-phase reactor for GLS reactions. Furthermore, according to FIGS. 1 and 2 in this article, the biological reaction takes place within the membranes/fibres; there is no reaction chamber located outside the fibres. This can also be seen from the fact that according to this literature citation the reaction mixture (that is to say the liquid phase) is both fed and removed via the hollow fibres, whilst according to the Application the hollow fibres provided in the reaction chamber serve only for removal of the liquid phase containing reaction products; the reactor is provided with a separate feed, connected to the reaction chamber, for the (liquid or dissolved) starting materials.
Furthermore, as can be seen from FIG. 4 of said article, the fibres are not located horizontally and/or transversely to the direction of the gas stream; it is therefore not possible to achieve transverse flow onto the fibres. Finally, neither the use of ceramic fibres nor the use of heat exchange elements provided in the reactor is described or suggested.
European Application 0 659 694 likewise describes a biological reactor, specifically for the treatment of polluted water. These reactor is provided with a gas lift system for circulating the liquid phase, the membrane elements arranged in the reactor, which open into the reaction chamber, appearing to serve solely for guiding the circulating liquid phase and not for removing the reaction products. Moreover, once again it is not possible to achieve transverse flow onto the fibres, whilst the use of ceramic fibres or heat exchange elements is also not described.
A general problem which confronts those skilled in the art when using membranes for GLS reactions is that, on the one hand, the solid phase--which preferably is in the form of small particles in order to provide as large as possible a contact surface--has to be held in suspension, so that said phase does not settle out; and that, on the other hand, said small particles must be prevented from clogging (the pores of) the membrane, which would impede the throughput of liquid medium through the reactor. This problem is intensified because during use of the reactor there will be a net liquid flow through the walls of the fibres, which flow "draws along" the solid particles to/against the membrane.
All of this is probably the reason why the use of hollow fibres in GLS reactors has to date not been proposed in the prior art. A further aim of the invention is, therefore, to offer a solution to these specific problems.